Indium that occurs in very small amounts in zinc ores is obtained as a by-product of zinc refining, and as the knowledge of its interesting properties has gradually spread, many proposals have been made as to its use. More recently, the high electric conductivity and transparency of thin films of indium oxides, especially the Sn-containing indium oxide (In.sub.2 O.sub.3 --SnO.sub.2) known as "ITO", have attracted much attention. This has led to the development of diverse applications, including liquid crystal display devices, thin-film electroluminescence display units, radioactive detector elements, and clear tablets of terminal equipments etc.
The indium oxide powder and tin oxide powder as starting materials for the manufacture of the above ITO films and others have hitherto been made, respectively, in the sequences of process steps shown at (a) and (b) in FIG. 6.
In the case of indium oxide powder, as shown in FIG. 6(a), the first stage starts with the dissolution of metallic indium with nitric acid, and the solution is neutralized with ammonia water to allow the product to settle as indium hydroxide. This precipitate or deposit (indium hydroxide) is then filtered, rinsed, and dried. Next, at the second stage, the resulting indium hydroxide is calcined or roasted to indium oxide powder.
For tin oxide powder, at the first stage in FIG. 6 (b), metallic tin is dissolved with nitric acid and allowed to settle as metastannic acid. The precipitate or deposit (metastannic acid) is filtered, rinsed, and dried. At the second stage, the metastannic acid so obtained is calcined or roasted to tin oxide powder.
The indium oxide powder and tin oxide powder thus prepared are often mixed and sintered to form a sputtering target or the like, which is then used as a material for forming a thin film, e.g., by sputtering.
However, it has been pointed out that the above processes for preparing indium oxide powder and tin oxide powder present the following problems:
a) The oxide powders thus obtained vary largely in their properties (average particle diameter, apparent density, etc.). This hampers the reduction of variance of qualitative uniformity or the improvement for higher quality of the ITO type display materials. PA1 b) The manufacturing conditions (solution temperature, reaction rate, etc.) are not always easy to control uniformly throughout. Stabilizing the conditions requires additional equipment cost. PA1 c) When a new need arises for powders dissimilar in properties to those currently in use, it cannot be met flexibly. PA1 d) The manufacturing equipment is relatively large in scale, and uniform control of the operating conditions requires considerable labor, and yet a requirement for increased production cannot necessarily be met with ease. PA1 e) Dissolution waste liquor (e.g., of ammonium nitrate) results from each cycle of process and has to be disposed of, adding to the running cost. PA1 (1) Indium hydroxide, a substance to be calcined or roasted to indium oxide, can be prepared under stabilized conditions at relatively low equipment and running costs if an electrolytic process is adopted in place of the dissolution,neutralization and settling process of the prior art. The indium oxide powder that results from the calcining or roasting has very broad acceptable ranges of various properties (including the average particle diameter and apparent density). Moreover, judicious choice of the electrolysis conditions permits fine control of those properties. PA1 (2) Metastannic acid, a substance to be calcined or roasted to tin oxide, can be prepared under stabilized conditions at relatively low equipment and running costs if an electrolytic process rather than the dissolution-settling process of the prior art is adopted. The tin oxide powder that results from the calcining or roasting has very broad acceptable ranges of various properties (including the average particle diameter and apparent density). Moreover, proper choice of the electrolysis conditions permits fine control of those properties. PA1 (3) If, in preparing mixed oxide powders of indium and tin, an electrolytic process is adopted instead of the conventional separate processes at the first stage, wherein indium and tin as separate anodes are simultaneously treated by electrolysis, then a mixed precipitate of indium hydroxide and metastannic acid can be directly prepared in a single step under stable conditions at relatively low equipment and running costs. Calcining or roasting of this mixed precipitate yields an ITO powder (a mixed powder of indium oxide and tin oxide powders) having very broad acceptable ranges of various properties (including the average particle diameter and apparent density) in a stable way. Those properties can be finely controlled through proper selection of the electrolysis conditions. PA1 (4) If, in preparing mixed oxide powders of indium and tin, an electrolytic process is adopted instead of the conventional separate processes at the first stage, wherein an alloy of indium and tin as an anode is treated by electrolysis, then a mixed precipitate of indium hydroxide and metastannic acid can be directly prepared in a single step under stable conditions at relatively low equipment and running costs. Calcining or roasting of this mixed precipitate gives an ITO powder having very broad acceptable ranges of various properties (including the average particle diameter and apparent density) in a stable way. Those properties can be finely controlled through proper selection of the electrolysis conditions. PA1 (1) a process for the preparation of an indium oxide-tin oxide powder which comprises electrolyzing indium as an anode to produce a precipitate of indium hydroxide, calcining or roasting the precipitate of indium hydroxide thus obtained to produce indium oxide, and combining the resulting indium oxide powder with a tin oxide powder; and PA1 (2) a process for the preparation of an indium oxide-tin oxide powder which comprises electrolyzing tin as an anode to produce a precipitate of metastannic acid, calcining or roasting the metastannic acid thus obtained to produce tin oxide, and combining the resulting tin oxide powder with an indium oxide powder. PA1 (3) a process for the preparation of an indium oxide-tin oxide powder which comprises simultaneously electrolyzing indium and tin as separate anodes to produce a mixed precipitate of indium hydroxide and metastannic acid and calcining or roasting the resulting mixed precipitate of indium hydroxide and metastannic acid to produce a mixed indium oxide-tin oxide powder; and PA1 (4) a process for the preparation of an indium oxide-tin oxide powder which comprises electrolyzing an alloy of indium and tin as an anode to produce a mixed precipitate of indium hydroxide and metastannic acid and calcining or roasting the resulting mixed precipitate of indium hydroxide and metastannic acid to produce a mixed indium oxide-tin oxide powder.